You've all been there: tired and exhausted, you drag your ass out for a run, but a few miles in—BOOM—your pain fades and you're suddenly euphoric. You've got a runner's high—and new research reveals that it's an evolutionary trait that fine-tuned us to be the ultimate hunter-gatherers.

The research, conducted by the University of Arizona and the University of Texas, shows that evolution has hard-wired us to perk up during endurance exercise. To do that, the body dumps neurotransmitters, called endocannabinoids, into our brain. Those compounds activate cannabinoid receptors—the same ones that are stimulated by cocaine—to provide that euphoric feeling during and after exercise.

The researchers tested out the theory that it's an evolved trait by studying the concentration of endocannabinoids in the blood of different animals exercising on treadmills for thirty minutes. In humans and dogs—animals which evolved for endurance exercise—the levels of the neurotransmitter rocketed. In animals that evolved to be sedentary—such as ferrets—there was no such increase. The results appaear in the Journal of Experimental Biology.

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The researchers point out that an ability to run long distances would have made us better hunter-gathers capable of getting more food—so natural selection would have ensured that those who experienced runners' highs all those years ago won out. These days, you're probably best off enjoying the feeling while it lasts. The day after is when the pain really starts. [Journal of Experimental Biology]